1. Field of the Invention
This invention relates generally to ventilators for delivering breathing gas to the lungs of a patient, and more particularly concerns a novel construction for a piston based ventilator used to deliver breathing gas to a patient.
2. Description of Related Art
Medical ventilators are widely utilized to provide breathing gas to & patient when the patient is unable to breath adequately without assistance. Such ventilators can be used for a wide variety of breathing strategies, including pressure assistance, which can be utilized when the patient has already begun an inspiratory effort, but is unable to fully complete the breath. One system in wide spread use for such ventilation is the piston type ventilator.
Early positive displacement ventilators utilized bellows systems to deliver a desired pressure and volume to a patient. These systems were quite effective, but had certain limitations which led to the design and construction of alternative mechanical constructions, including piston type ventilators.
Piston type ventilators known in the art commonly use a piston assembly wherein a piston head slides over a fixed central shaft. The gas to be delivered is drawn into cylinder through an inlet valve by the negative pressure created during the retraction of the piston and is subsequently delivered to the patient through an outlet valve by advancing the piston. In such a system, the volume of air delivered is directly related to the piston displacement within the cylinder. Conventionally, the piston head is connected to one or more piston rods which are driven by a motorized system to cause the piston head to move longitudinally over a fixed central shaft within the cylinder.
Piston lung ventilators known in the art also typically contain piston rings, usually a metal plastic or composite ring for sealing the gap between the piston and the cylinder wall. In order to avoid excess friction, these piston rings frequently do not make a tight seal to the cylinder wall, allowing breathing gas with a high oxygen concentration to escape. For example, in a typical piston ventilator, the piston cylinder has a volume of about 2.7 liters, to allow for a loss of about 0.7 liters by leakage past the piston and compression of gas in the cylinder and in the delivery system, in order to still deliver up to 2.0 liters of breathing gas to the patient in one stroke of the piston. Due to the risk of fire, it is unsafe to allow oxygen enriched air to accumulate in the interior of an electrical product such as a ventilator. The breathing gas mixture to be delivered to the patient can also be diluted by the leakage, or `blow by`, of room air that leaks past the piston rings during the time the piston is retracting in order to draw breathing gas into the cylinder Furthermore, the friction inherent in the use of piston rings requires more energy to operate the piston and increases stiction and friction of the piston-cylinder assembly.
Additionally, known piston ventilator designs usually contain one or more of the following elements,. multiple linear bearings and seals located in the cylinder and plates that can, after wear, cause angular displacement and eventual tilt and interference between the piston and the cylinder; several stages of connection between the motor, piston rod and piston head which can cause backlash due to tolerance build up during operation of the system; bearing housings contained within the cylinder which decrease the volumetric capacity and piston displacement capability of the unit by reducing the stroke length of the piston head and attached rod. It can be understood that a piston for displacement of air that is self jigging to reduce tilt and interference, that would reduce the number of connections between the motor and the piston head and that would be a cost effective and easy to manufacture alternative to devices currently available would be novel in the art. The present invention meets these needs.